The present invention relates to image-processing apparatus for processing image data, image-capturing apparatus equipped with the same, image-processing method and image-processing program.
At present, the digital image data acquired by scanning a color photo-film or the digital image data captured by an image-capturing apparatus, such as a digital camera, etc., is distributed through such a memory device as a CD-R (Compact Disk Recordable), a flexible disc and a memory card or the Internet, and is displayed on such a display monitor as a CRT (Cathode Ray Tube), a liquid crystal display and a plasma display or a small-sized liquid crystal monitor display device of a cellular phone, or is printed out as a hard copy image using such an output device as a digital printer, an inkjet printer and a thermal printer. In this way, displaying and printing methods have been diversified in recent years.
However, since color regions and/or gradation characteristics established for various kinds of displaying/printing devices are different from relative to each other corresponding to principles and internal configurations employed in the displaying/printing devices, there have been many cases that an “appearance” of image reproduced on the basis of the same image data varies with a wide variety of displaying/printing methods to be employed. To eliminate such the variation of the “appearance”, many efforts have been implemented so far.
For instance, there has been an attempt that a color space represented by RGB digital signals is standardized into another color space, which is independent of characteristics of the image-capturing apparatus. At present, the sRGB (refer to “Multimedia Systems and Equipment—Color Measurement and Management—Part 2-1: Color Management—Default RGB Color Space—sRGB” IEC61966-2-1) have been employed for most of digital image data as a standardized color space. The color space of this sRGB has been established to meet the color reproduction area for a standard CRT display monitor.
However, the color reproduction area of an image displayed on the displaying device, such as a CRT display monitor, etc., or that of a hard-copy image printed by one of various kinds of printing devices varies with a fluorescent material or a combination of dye materials to be employed. For instance, the color reproduction area, reproduced by the CRT display monitor corresponding to the sRGB standard color space, includes a wide area of bright green and blue, and an area which could not be reproduced by a silver-halide print, an ink-jet print and a printed hard-copy. Conversely, a cyan area reproduced by the ink-jet print and the printed hard-copy or a yellow area reproduced by the silver-halide print includes an area which could not be reproduced by the CRT display monitor (for instance, refer to “Fine Imaging and Digital Photograph” p. 444, edited by Publishing Committee of Society of Photographic Science and Technology, Japan, published by Corona Co.).
To cope with the problem mentioned in the above, the ICC Profile Format specified by the ICC (International Color Consortium) has been frequently employed so far. The method according to the ICC Profile Format includes the steps of: attaching first color space information of a first device to first image data, which are generated under an intention of rendering the first device to reproduce an image based on the first image data; when a second device is employed for reproducing the image, converting the first image data to third image data in the PCS (Profile Connection Space) color space, which is independent of a specific device, based on the first color space information; and in the next step, converting the third image data to second image data suitable for the image reproduction by the second device, based on the second color space information for the second device. According to this method, the color reproducibility of colorimetry values measured by the colorimeter between the color reproducing devices has been considerably improved.
It is well known, however, that there have been many cases that, even when the colorimetry values are precisely reproduced, the “color appearance” is different for the real viewer. This is because, the human eyes do not sense an absolute colorimetry value as his visual sense, but changes the “color appearance” in its adaptation state corresponding to viewing conditions (such as brightness of a peripheral area, background, etc.). For instance, since the general viewing conditions and adaptation states of an image reproduced by an illumination displaying device like the CRT and that formed on a reflection displaying material like the printed matter are different from each other, there are many cases that the “color appearances” of them do not coincide with each other, even if the colorimetry values of them coincide with each other.
To cope with the problems mentioned in the above, there has been well known the color managing method, which employs a color appearance model. The color appearance model is such a model that is used for predicting the “color appearance” under a wide variety of the viewing conditions. Concretely speaking, the value representing the “color appearance” under a designated condition is derived from the colorimetry values by conducting a conversion based on the viewing condition parameters. For instance, CIECAM97s, which was recommended as a standard model by the CIE (International Commission on Illumination), has been frequently employed as such the color appearance model. Further, the CIE will issue a recommendation of the CIECAM02 as an improved version of CIECAM97s, later soon (for instance, refer to Non-patent Document 1).
Other than the above, various kinds of the color appearance models, such as the Noya model, the Hunt model, the RLab model, the LLab model, etc., were already announced so far. As concretely examples of the viewing condition parameters to be employed for the calculations in such the color appearance models, the adapting field luminance, the tristimulus values of white in the adapting field, the relative luminance of the source background, the impact of surround, etc. can be cited. FIG. 13 shows concrete viewing condition parameters. For instance, the concrete viewing condition parameters as cited in FIG. 13 are defined in the CIECAM97s.
When employing such the color appearance model, for instance, the first image data, which are created with the intention of reproducing its image under the first viewing condition, can be converted to the values representing the “color appearance” by applying the first image data to the color-appearance model transform on the basis of the first viewing condition parameters corresponding to the first viewing condition. Then, the values representing the “color appearance” can be converted to the second image data, which is intended to reproduce its image under the second viewing condition, by applying the values representing the “color appearance” to the color-appearance model inverse-transform on the basis of the second viewing condition parameters corresponding to the second viewing condition. According to the method mentioned in the above, it becomes possible to make the “color appearance” under the first viewing condition and that under the second viewing condition coinciding with each other, even if the first viewing condition and the second viewing condition are different form each other (for instance, refer to Non-patent Document 1).
As described in the above, to implement the color management employing the color appearance model, it is necessary to input the viewing condition parameters corresponding to the viewing condition intended, as well as the image data.
Patent Document 1                Tokkaihei 7-222196 (Japanese Non-Examined Patent Publication)        
Non-patent Document 1                “COLOR APPEARANCE MODEL—SUMMARY AND PROBLEM OF CIECAM02” by Hirohisa Yaguchi, ABSTRACT COLLECTION OF THE COLOR FORUM 2003 OF THE INSTITUTE OF IMAGE ELECTRONICS ENGINEERING OF JAPAN, 2003, P. 57        
Incidentally, when image data is generated by conducting an image-capturing operation by means of, for instance, a digital camera, it is necessary to determine the viewing condition parameters corresponding to the image data in advance, in order to apply the color management employing the color appearance model to the image data. It would be a problem how to establish the viewing condition parameters.
It is cumbersome to measure the viewing condition parameters (such as the adapting field luminance, the tristimulus values of white in the adapting field, the relative luminance of the source background, the impact of surround, etc.) of the scene observer, who is present at the captured scene, every time when capturing the scene. Specifically, as for a high contrasted image, the viewing condition parameters depend on a place where the observer's notice is directed. It is difficult, however, to measure a visual axis of the observer, every time when capturing the scene.
Further, when the viewing condition parameters are intended to be attached to the “image data representing a visual image”, the luminance of the scene should be converted to that of the visual image according to the viewing condition intended for the visual image, and, in addition, it is necessary to calculate concrete viewing-condition parameters, which strictly represent the viewing condition intended for the visual image.
As mentioned in the above, the operation for attaching appropriate viewing-condition parameters to the image data is difficult task, and therefore, it is virtually impossible especially for an operator who has no specific knowledge about the color appearance model to calculate such the appropriate viewing-condition parameters.